This has been a issue for ages.
Attempting to read the BLS, you will see these lines:
"+-OWNER (Insert BLID here)"
Each one of those bricks seem to be connected to the ground plane:
Here's an example:
+-OWNER 18701
8x8F" 113 230.5 0.1 0 1 55 0 0 1 1 1
+-OWNER 11977
8x8F" 125 226.5 0.1 0 1 55 0 0 1 1 1
+-OWNER 11977
8x8F" 121 226.5 0.1 0 1 54 0 0 1 1 1
+-OWNER 18701
8x8F" 125 234.5 0.1 0 1 55 0 0 1 1 1
+-OWNER 11977
8x8F" 121 234.5 0.1 0 1 54 0 0 1 1 1
+-OWNER 18701
8x8F" 121 238.5 0.1 0 1 55 0 0 1 1 1
+-OWNER 11977
8x8F" 113 238.5 0.1 0 1 55 0 0 1 1 1
+-OWNER 11977
8x8F" 113 234.5 0.1 0 1 54 0 0 1 1 1
+-OWNER 18701
2x8F" -41 80 0.1 1 1 42 0 0 1 1 1
+-OWNER 1940
8x8F" 121 230.5 0.1 0 1 55 0 0 1 1 1
+-OWNER 11977
6x16F" -41.5 75.5 0.1 0 1 42 0 0 1 1 1
+-OWNER 1940
8x8F" 125 230.5 0.1 0 1 54 0 0 1 1 1
+-OWNER 11977
8x8F" 117 238.5 0.1 0 1 54 0 0 1 1 1
+-OWNER 18701
8x8F" 117 234.5 0.1 0 1 55 0 0 1 1 1
Note the 0.1 in about midway to each line.
And then there are the normal bricks.
1x10F" 111.75 232.5 0.3 0 0 36 0 0 1 1 1
1x8" 111.25 232.5 0.5 0 0 36 0 0 1 1 1
2x4" 108 240 0.5 1 0 4 0 0 1 1 1
2x10" 100.5 240 0.5 1 0 4 0 0 1 1 1
1x2F" 111.5 229.25 0.3 1 0 36 0 0 1 1 1
4x12F" 108 246.5 0.3 0 0 14 0 0 1 1 1
6x12F" 106 242 0.3 1 0 14 0 0 1 1 1
1x1F" 111.25 230.25 0.3 0 0 36 0 0 1 1 1
1x2F" 111.5 229.75 0.3 1 0 36 0 0 1 1 1
2x2F" 96.5 250 0.3 0 0 14 0 0 1 1 1
4x4F" 97 248.5 0.3 0 0 14 0 0 1 1 1
2x4F" 97.5 251.5 0.3 0 0 14 0 0 1 1 1
2x2F" 97.5 250 0.3 0 0 14 0 0 1 1 1
2x10" 96.5 229 0.5 1 0 4 0 0 1 1 1
16x16 Base" 86 248.5 0.3 0 0 4 0 0 1 1 1
1x16F" 81.75 248.5 0.3 0 0 14 0 0 1 1 1
1x16F" 90.25 248.5 0.3 0 0 14 0 0 1 1 1
2x2F" 97.5 253 0.3 0 0 14 0 0 1 1 1
1x16F" 90.75 248.5 0.3 0 0 14 0 0 1 1 1
1x16F" 81.25 248.5 0.3 0 0 14 0 0 1 1 1
1x1x5" -57.75 82.25 1.7 0 0 43 0 0 1 1 1
1x2" -56.25 83 0.5 0 0 43 0 0 1 1 1
1x1" -56.75 82.75 0.5 0 0 43 0 0 1 1 1
None of these have 0.1 and I guess thus it can safely be assumed that specific part with OWNER is defining all bricks that are on the ground plane, and they determine the ownership.
Instead of the alternative listed in the OP about putting ID after each brick, I don't understand why can't we simply have them categorized on the already existing ownership definers like so.
+-OWNER 18701
8x8F" -81 246.5 0.1 0 1 50 0 0 1 1 1
1x4x5" -10.75 204.5 11.9 0 0 39 0 0 1 1 1
1x2" -8.5 201.25 12.5 1 0 39 0 0 1 1 1
1x2" -10.25 203 12.5 0 0 39 0 0 1 1 1
1x2" -10.25 206 12.5 0 0 39 0 0 1 1 1
1x2" -8.5 207.75 12.5 1 0 39 0 0 1 1 1
+-OWNER 32634
8x8F" -25 266.5 0.1 0 1 49 0 0 1 1 1
1x1x3" -6.25 200.75 10.1 0 0 39 0 0 1 1 1
1x1x3" -7.75 200.75 10.1 0 0 39 0 0 1 1 1
1x2x5" -5.5 201.25 10.7 1 0 39 0 0 1 1 1
1x1x5" -4.75 201.75 10.7 0 0 39 0 0 1 1 1
1x1x5" -4.25 202.25 10.7 0 0 39 0 0 1 1 1
1x1x3" -3.25 203.75 10.1 0 0 39 0 0 1 1 1
1x2x5" -3.75 203 10.7 0 0 39 0 0 1 1 1
+-OWNER 7643
8x8F" -25 270.5 0.1 0 1 50 0 0 1 1 1
1x4" -3.25 204.5 8.9 0 0 39 0 0 1 1 1
1x1" -3.75 205.25 8.9 0 0 39 0 0 1 1 1
1x4 Arch" -3.25 204.5 11.3 2 0 39 0 0 1 1 1
1x1x3" -3.25 205.25 10.1 0 0 39 0 0 1 1 1
1x2x5" -3.75 206 10.7 0 0 39 0 0 1 1 1